In a communications system, a radio frequency power amplifier and a microwave power amplifier are used to amplify a modulated radio frequency signal and a modulated microwave signal. To improve efficiency of spectrum utilization, modulation signals in many standards carry both phase information and amplitude information. For example, a Code Division Multiple Access (CDMA) signal, an orthogonal frequency division multiplexing (Orthogonal Frequency Division Multiplexing, OFDM) signal, and the like are non-constant envelope signals having a high peak-to-average ratio, and such signals require that a power amplifier not only has relatively good linearity at a peak output power, but also has relatively high efficiency at an average output power. A conventional class AB linear power amplifier has high efficiency only at the peak output power, and for a non-constant envelope signal having a high peak-to-average ratio, the conventional class AB linear power amplifier has very low efficiency at the average power.
To improve efficiency of a linear power amplifier during power back-off, a Doherty power amplifier is put forward. A common Doherty power amplifier may include at least two power amplifiers, where one power amplifier is a main power amplifier (Main Amplifier), which is also referred to as a carrier power amplifier, and is biased to a class AB working state; and the common Doherty power amplifier further includes one or more auxiliary power amplifiers (Auxiliary Amplifier), which are also referred to as peak power amplifiers (Peak Amplifier), and are biased to a class B or class C working state. In a case in which an input signal is less than a specified threshold, the auxiliary power amplifier does not achieve a function of amplifying the signal. Because of a function of a Doherty combiner network, when the auxiliary power amplifier does not achieve an amplification function, output impedance of the main power amplifier is in a high-impedance state. In this way, the main power amplifier can be saturated at a relatively low output power, thereby improving efficiency at the low output power. In an ideal state, when the input signal is higher than the specified threshold, an output power of the auxiliary power amplifier increases as an input power increases, which achieves an active-load pulling function for the main power amplifier. Output impedance of the main power amplifier decreases as an input power increases, and an output power increases as the input power increases. A working state of the main power amplifier is always in a saturated state, until the main power amplifier and the auxiliary power amplifier finally achieve saturated output simultaneously, thereby improving efficiency of an entire power amplifier during power back-off.
However, because a gain feature of a power amplifier biased to a class C working state is slowly turned on, before a power of an input signal of a main power amplifier reaches a specified threshold, an auxiliary power amplifier already starts to perform active-load pulling on the main power amplifier. As a result, the main power amplifier does not reach a saturated state when the input signal reaches the specified threshold, thereby affecting efficiency of the power amplifier.